1. Field of the Invention
This invention relates to a brushless direct current electric motor. More particularly, it relates to an improvement in a winding structure in an ultra small size direct current motor of a longer length or in a reduced thickness large diameter motor.
2. Prior Art
A number of various constructions have been suggested and adopted for kind of the brushless direct current electric motor; typical of these is a construction shown in FIGS. 1 and 2, although it is not shown herein in what reference material or literature it is introduced.
Referring to FIGS. 1 and 2, the numeral 1 denotes a casing which is substantially cylindrical in its entirety and provided at the axial end parts thereof with a pair of bearings 2, 2 a substantially cylindrical stator 3 formed by a large number of laminated stator core pieces 3a as shown in FIG. 2 is disposed in contact with the inner wall 1a of the casing 1.
A rotor 4 as later described is disposed for rotation within a central axial bore 5 of the stator 3, plural magnetic poles 6 are formed radially for defining the peripheral wall of the bore 5, and a driving coil holding recess 7 is defined between adjacent ones of these magnetic poles 6, 6.
A driving coil 8 is disposed within these driving coil holding recesses 7. The driving coil 8 may be wound by an automatic winding machine (not shown), or the coil may be previously wound and subsequently inserted into the coil holding recesses 7.
A rotary shaft 9 formed integrally with the rotor 4 is disposed for rotation within the bearings 2, 2, with the driving electric current flowing from an electrical source, not shown, through an electronic switching means, not shown, and through the driving coil 8.
The brushless direct current electric motor of the prior art described above, has had the following disadvantages.
(i) In the case of the ultra small size direct current electric motor with a longer length, the stator diameter is not larger than about 20 mm, so that the central axial bore formed in the stator for accommodating the rotor and the space available for forming the drive coil holding sections are extremely small, such that extreme difficulties are encountered in the operation of performing the winding by an automatic winding machine or in inserting the wound drive coil, with the result that production efficiency is considerably lowered.
(ii) In case of a longer stator length, such as more than about 30 mm, extreme difficulties are encountered in the operation of inserting the driving coil, such that it becomes virtually impossible to obtain a motor of a size less than a predetermined small size of the ultra-small direct current electric motor.
(iii) On the other hand, the stator of an electric motor of a reduced thickness and a large diameter, such as the direct drive electric motor, is larger in diameter and hence of the multi-pole multi-slot type structure. With the increase in the number of slots, however, the number of the coils is also increased, with consequent increase in working time.
In view of the above described disadvantages of the prior art motor, it is a principal object of the present invention to provide a direct current electric motor of an ultra small size and a longer length and an electric motor of a large size and reduced thickness, wherein the stator is formed of a plurality of stator elements to permit an extremely facilitated coil winding operation.
According to the present invention, there is provided a brushless direct current electric motor comprising a casing, a stator and driving coils disposed within said casing, and a rotor having a plurality of alternating north and south magnetic poles formed on the outer periphery of the rotor, said rotor being disposed within said casing, wherein the stator is composed of a plurality of stator elements, and driving coils are wound separately on the respective ones of the stator coil elements.
In the brushless direct current electric motor of he present invention, since the stator is composed of a plurality of stator elements, the driving coils can be applied separately to the respective stator elements, these stator elements, thus wound with the driving coils, being then inserted into the casing. In such a manner, it becomes readily possible to produce the stator with the driving coil applied thereto even in the case of a direct current electric motor of a longer length and a reduced diameter, for a multi-pole stator, it is only necessary to increase the number of magnetic poles formed on the stator elements, it being unnecessary to increase the number of the coils with the increase in the number of magnetic poles. In such a manner, it becomes readily possible to produce a multiple pole large stator type electric motor.
Also, by reason of the above described stator structure, it becomes readily possible to produce a direct current electric motor of a longer length and an ultra-small overall size which it was impossible to produce in the prior-art practice.
In short, the brushless direct current electric motor of the present invention has the following advantages.
Since the stator is formed by a plurality of stator elements that can be wound separately with driving coils, the operation of winding the coil about the stator may be facilitated, such that the driving coil can be applied easily, even in cases wherein the stator is reduced in diameter and increased in axial length.
Also, even in these cases, the stator may be produced as desired, subject to selection of the number of the stator elements, such that the difficulties in production of an electric motor having a small diameter and a longer length may be substantially eliminated. Also, since each stator element may be assembled by connecting a pair of stator pieces to each other, the axial length of each stator element can be changed freely to produce the stator of a direct length conforming to the axial length of the rotor. In the direct drive electric motors, such as multi-pole motors with a reduced thickness and a larger diameter, there is no necessity of increasing the number of coils with the increase in the number of magnetic poles so that the work load is not increased, and hence the manufacture of the stator is also facilitated.